Radiation detectors



Aug. 14, 1956 R. J. RUBLE RADIATION DETECTORS Filed Dec. 17, 1951 INVENTOR.

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TOENEXE' RADIATION DETECTORS Raymond J. Ruble, Beacon, N. Y., assignor to Texaco Development Corporation, New York, N. Y., a corporation of Delaware Application December 17, 1951, Serial N 0. 262,029

6 Claims. (Cl. 313-93) This invention relates to improvements in detectors of penetrative radiation. More particularly it relates to improvements in Geiger-Mueller types of such detectors.

A number of prior art developments of Geiger-Mueller detectors have been directed at minimizing the effects of two limitations: (1) that, as such, relatively 'few interactions of penetrative radiationsv occur in the voluminous gaseous filling of the detector; and (2) that the ionizing particles produced by interactions, e. g., Compton or photo-electrons, are usually not nearly so penetrative as the original radiation. Because of the first limitation the gas, which admittedly is very useful in counting interactions, does not add to the total number thereof to any significant extent. For example, if an intercepted gamma ray should interact with the detector at all it is usually with some denser material thereof, such as its metallic cathode, rather than with its gaseous filling. Of course the way that the interaction gets to be counted does in volve the gas, i. e., the interaction produces, as by-products, one or more charged particles within the dense material, at the same time transferring some or all of its atent C energy thereto, and these particles, by escaping into the gas, may ionize it and thereby start an electron avalanche.

Because of the second limitation special constructions have had to be used. For example in the original Geiger- Mueller tubes the cathode walls had to be of a thickness which represented a compromise between two extremes: (1) if they were quite thin, to assure facile escape of ionizing particles, then most of the highly penetrative radiation impinging on the detector would pass right through without being involved in interactions; (2) on the other hand if they were quite thick, to increase the incidence of interactions, then most of the ionizing particles would not escape into the gaseous filling.

The above-mentioned prior art developments have resulted in: (a) providing the tube with increased amounts of dense material, without increasing its volume, by arrangements using complex multi-element cathodes; (b) forming and positioning the cathode elements to optimize the incidence of interactions without impeding the escape or" the particles produced thereby; and (c) arranging these elements so that suitable discharge paths extend to the anode(s) from regions, adjacent the surfaces of the cathode elements, into which the particles are likely to escape.

A number of improved Geiger-Mueller detectors embodying these developments are shown in U. S. Patent 2,397,071-3 and illustrate how the cathode elements can be positioned so that the large amount of dense material which they comprise can increase the average number of interactions without unduly interfering with the escape of the charged-particle by-products of the interactions and also without some of the elements electrically shielding one or more of the others from the electron-collecting field of the anode(s). In these detectors the cathode elements have various wafer-like shapes and each of them is thin enough to permit a charged-particle to escape into the gas it its initial velocity is toward one of the surfaces of the element. In addition they are positioned in the detector so that their greatest exposure to the source of radiation is edgewise. As a result, as shown in Fig. 4 of the U. S. Patent 2,397,071, and also in Fig. 1 herein, a gamma ray which enters the edge of a cathode element, or enters one of its sides at a grazing angle, will have a long path through dense material and therefore a considerable likelihood of sustaining an interaction. Moreover a charged particle produced by such an interaction will more often than not move oil in an entirely diiferent direction than that of the radiation which produced it, i. e., in a direction having a large component along a normal path with respect to one of the exposed surfaces of the cathode element. Therefore such structures increase the percentage of interactions and at the same time permit a high percentage of the ionizing particles to escape into the gas.

Provision is made for many of the escaped particles, and/or secondary electrons which they produce in the gas, to be drawn to the anode(s), to thereby start electron avalanches, by positioning the anode(s) where both surfaces of each cathode element can see some portion thereof along a straight line which is unobstructed by any other cathode element(s), e. g., by mounting each thin-wire anode so that it extends crosswise to, and through aligned holes of, a succession of cathode elements.

While these improved arrangements of the prior art eliminated the need for compromising between the two extremes mentioned above, in order to select an optimum cathode wall thickness, they gave rise to a new need for compromising between extremes, in this case to select an optimum inter-element spacing. If as one extreme, excessively small inter-element spacings were used, e. g., with a view to increasing the number of cathode elements containable within a given detector and thereby increase the incidence of interactions, then the electron-collecting field of the anode(s) would not be able to penetrate deeply enough into the inter-element spacings to draw out most of the escaped charged particles and/or the secondary electrons which they produce in the gas. Because of this many of these particles would not be able to start Townsend avalanches and the interactions which produced these particles would go uncounted. If, as an opposite extreme, excessively large spacings were used, e. g., to increase the efiiciency of the device for collecting escaped charge particles and/or the secondaries which they produce, then the incidence of interactions would be reduced.

Moreover, while it might seem that the collection of charged particles in a detector which appeared to have excessively close spacings between its cathode elements could easily -be increased by increasing the anode-tocathode potential, this has not been available as a satisfactory practical expedient. One reason for this is that certain anode potentials should not be exceeded if one is to obtain certain kinds of operation, e. g., proportional counting. Another reason is the possibility of cold emission from the edges of the cathode elements which face toward the anode(s) if an excessive anode-to-cathode potential is used.

Similarly, while the collection of charged particles might be increased by increasing the number of anode wires which pass through the stack of cathode elements (see Fig. 3 of U. S. Patent 2,397,071 and Fig. 2 herein), it should be borne in mind that each time that a hole is made in a cathode element it reduces the total amount of dense material comprised therein and thereby reduces the probable incidence of interactions.

Accordingly it is an object of the present invention to provide improvements in Geiger-Mueller radiation detectors of the kind described above whereby one may use unusually small spacings" between adjacent cathode elements and yet attain unexpectedly high efiicieney in collecting the charged-particle by-products of interactions and/ or the secondary electrons produced thereby.

Itis a further object or the present invention to also improve the collection emeiney of prototype of Geiger- Mu'eller detectors;

In general these objects are attained by including in the detector one or more additional electrodes for providing, in regions) into which charged particles are likely to escape and iii which their secondaries are likely to be produced,- stronger collecting fields than those which normally would be provided therein as a result of the potential applied between the cathode and the fine-wire an'ode(s). As is known there is a ver great coii'centr'a tion of the available field about a Geig'e'f Mue'ller anode. Because of this the field gradie'n'ts' which exist near to portions ofthe cathode structure are relatively very low. According to the present invention increased electron collecting fields areestabli'she'd in the regions in question by the use of one or more additional electrodes whereby escaped charged particles and/or their secondaries are forcibly drawn from these regions and projected into the fie1d(s) of the anodfs) wherein they receive their final acceleration for producing Townsend avalanches.

It will be seen that anincidental advantage attained by the present invention is the provision of a Geiger- Mueller detector whose efiiciency tor the detection of certain penetrative radiation is selectively adjustable whereby it affords a convenient way of discriminating between ditterent kinds of radiation.

In the drawing:

Fig. 1 represents a longitudinal section through a multiple-plate Geiger-Mueller detector embodying improvement features of the present invention;

Fig. 2 is an end view of another multiple-plate G. M. tube also embodying improvement features of the present invention;

Fig. 3 represents a longitudinal section of a portion of a modification of the embodiment of Fig. l; and

Fig. 4 represents a longitudinal section through a prototype Geiger-Mueller tube embodying an improvement feature of the present invention.

The detector 10 shown in Fig. 1 comprises aplurality of cathode elements 11- positioned in parallel spaced relationship and having at least one row of aligned apertures 12 through which extends a fine wire anode 13. This portion of the structure of the detector 10 is in accordance with the prior art of so-called multiple-plate detectors.

It is to be understood that the detector 10- should include certain necessary structural features such as means for supporting the cathode elements 11 and electrically interconnecting them a gas-tight envelope, anda filling of ionizable gas. However since these features do not constitute essentialparts of the present invention, they are not shown or described in detail herein. The thickn'esses and proportions of the various elements which appear in the drawing and the spacings between them have been chosen for simplicity and clarity and are not intended necessarily to be representative of actual dimensiori's. According to the prior art the spacing-s between cathode plates of any given diameter cannot be reduced beyond a certain point without adversely efiecting the collection of charged particles as explained above. For example, it has been empirically determined that cathode elements having two inch diameters cannot be advantageously spaced any less than e36 of an inch apart. However by using one or more auxiliary electrodes, as roposed herein for the specific purpose of setting up strong collecting fields in the interelement spacings, the spacings of the cathode plates can be made smaller than was previously possible; v

The embodiment of Fig. 1 employs two-substantially cylindrical auxiliary electrodes, arefiector 14 and apreaccelerator 15, the first of which surrounds the outside of the stackof cathode elements 11 and the other of which fits within their row of aligned apertures 12. Each of these electrodes is in spaced and insulating relationship to the multiple-plate cathode so that it can be appropriately polarized with respect thereto for accelerating electrons through the inter-element spaces radially inward towards the anode 13. V p

In an embodiment of this kind any negative charged particles which are produced in the inter-element spaces will be urged in the direction ofthe anode 13 by the combined efiect of being pushed by the reflector, so to speak, and pulled by the pre-accelerator. Some electron multiplication will occur even before the charged particles pass through the pre+accelerator electrode 15, on their way to the anode 15, provided the pre-accelerating potential is substantially. greater than the ionization potential of the gaseous filling of the detector. Therefore by making this potential large er'iou'ghg the fact that some electrons unavoidably virillbecollected by the electrode 15" should not adversely sneer the countingeff ciency.

The inside surface of the reflector electrode 14, shown by way of example in Fig. 1,- is shaped with a respective convex angular groove 16 facinginto each of the interelectrode spaces of the cathode. The purpose of this is to cause the equi'potential surfaces which will be set up" between the reflector and the stack of cathode plates during operation to have appropriate configurations to direct the charged particles convergently toward the centers ofthese spaces rather than divergently toward the surfaces of the cathode elements 11.

As is known the ionizing potential which is essential tor providing electron multiplication during the development of a pulse must be effectively eliminated immediate- 1y thereafter so that the detector can recover and therefore be readied for a new count. Otherwise the potential which usefully accelerates electrons toward the anode will also eventually accelerate the heavier positive ions back onto the cathode thereby producing secondary electrons and sustaining a continuous discharge. It is to avoid this that a large quenching resistor 8- is usually employed in series with the cathode-to-an'ode energizing source. During a pulse it drops the applied potential to sucha low value that the weak fields which remain within the detector cannot sufficiently accelerate the positive ions and free electrons to prolong the Townsend avalanche which produced these particles. Therefore all further ionization ceases and these residual particles are free either to defuse thermally to the side walls, and similar interior surfaces of the tube where they can be absorbed in recombinations, or to get there by a combination of thermal ambi polar difiusion and the aiding efiect of being swept (in opposite directtions and at lower than ionizingv velocities) by and through these weak fields; Fromthe foregoing it will be seen that the quenching which is required for this type of tube in the regions between its cathode and each of the electrodes 14 and ,15 can be accomplished by a means (represented by the block 26' in Fig. 1) which can either .efle'ctively eliminate all of the electric field between each of these electrodes and the cathode or can reduce it to a lower-than ion-i'zing value:

Asnoted above during the pro-acceleration of the charged particles considerable electron multiplication can occur and a significant amount of the electron current ma be collected on the re accneramr grid. Thus anfauxilia'ry quenching resistor may be used as the required means, the return current of this electrode being relied upon to produce the required voltage drop. However it is possible, in this arrangement, that the recharging time constantfor the auxiliary electrode may be larger than desired, due, for example; to the capacitance between this electrode and the cathode and/ or the value of the quenching resistor is required. Accordingly avsai is an electronic switch may be employed as an alternative and faster-acting means. Such a switch may be actuated by each output current pulse of the detector to clamp the preaccelerator grid 15 to any desired potential level such as the potential of the cathode or a potential which is only sufliciently above it to provide non-ionizing sweeping fields. Accordingly an optional connection between the quenching means and the fine wire anode 13 is represented, by a dotted line 17, in Fig. 1.

If desired the quenching means 16 may be dispensed with entirely by the simple expedient of using lowerthan-ionizing pre-accelerating potentials.

Since the reflector electrode 14 will ordinarily not collect or emit enough electrons to have a return current of any significant magnitude it either should be operated with a fixed reflector voltage which, as aided by the pre-accelerator voltage, is insuflicient to produce ionization or with a dynamically changing voltage provided through and controlled by an electronic switch as represented at 18, 19.

The embodiment of Fig. 2 is of modification of that of Fig. 1 in which each cathode plate 11 comprises a plurality of symmetrically located holes rather than a single one at the center of the plate. Multiplier plate detectors having such cathode plates are known to the art as is shown in the U. S. Patent 2,397,071 mentioned above. In this embodiment each of the fine wire anodes 13 is surrounded by a pre-accelerator grid 15 like that of the Fig. 1 embodiment and the entire cathode structure is surrounded by a reflector 14' which is like that of Fig. 1 but of larger diameter. In the modified geometry of this embodiment the push-pull cooperation of the reflector with any one of the pre-accelerators is not so uniform as in the simpler arrangement of Fig. 1. However the reflector 14 nevertheless is very useful. As shown in Fig. 1 it is possible for a single quantum 20 of penetrative radiation to enter one side of the detector and not have an interaction until it has penetrated one of the cathode elements to a point thereof close to its opposite side and for a charged-particle by-product of the interaction to have its initial velocity in a direction and a region to carry it entirely out of the multiple-plate cathode. One of the useful functions of the reflector 14 is to reflect into the cathode any such particle and/ or any secondary electrons produced by it so that they can be picked up by the field of a pre-accelerator and drawn toward an anode.

The embodiment of Fig. 3 employs modified reflector and pre-accelerator elements each comprising a plurality of rings, 22 and 24 respectively. Each of the pre-accelerator rings 24 is small enough to fit within the aperture of a cathode element without contacting it. On the other hand each of the reflector rings 22 is in the form of a larger diameter ring which occupies the peripheral portion of the space between a respective pair of adjacent cathode elements. In this embodiment the collection of charged particles by the pre-accelerator will be very much reduced. In fact it is very likely that its return current will be too small to permit the use of a series resistor as a quenching means.

The embodiment of Fig. 4 is a prototype Geiger- Mueller tube also embodying the present invention. Although these tubes often inherently have higher efiiciencies for for the collection of charged particles than do multiple-plate detectors, nevertheless this efficiency can be made even higher through the use of an auxiliary electrode, i. e., the pre-accelerator of Fig. 4, for preaccelerating the negative-charged particles towards the anode. Statistically of all the interactions which occur in the cathode 25 and cause charged-particle by-products to escape into the interior of the tube, some will cause these particles to escape with initial velocities which are crosswise to radii of the tube, i. e., are along chords of the cylindrical cathode. Therefore these particles will go right back to the cathode and, on teaching it, some of them will be absorbed.

However by using the pre-accelerator 15 and applying a potential of relatively small magnitude between it and the cathode, one can set up a field having a sufficient gradient to draw some of these electrons away from the cathode and cause them to move inward toward the anode.

As mentioned above such a gradient cannot be provided near the cathode by simply increasing the cathode-to-anode potential since most of any increase will be effective in a region near to the anode.

An incidental advantage of building this kind of a G. M. tube is that by using a negative, instead of a positive auxiliary electrode-to-cathode bias and by making this bias variable, the efliciency of the tube will be made adjustable. In fact, the tube may be made to have selectivity between dilferent kinds of radiation by adjusting it to have zero efliciency for one of them and some finite value of efliciency for the other.

Thus this kind of detector can be adjusted so as to count substantially no gamma interactions while at the same time it continues to produce a count for almost every through-and-through penetration of the detector by a cosmic ray.

Obviously many modifications and variations of the invention, as hereinbefore set forth may be made without departing from the spirit and scope thereof, and therefore only such limitations should be imposed as are indicated in the appended claims.

I claim:

1. A radiation detector of the Geiger-Mueller type comprising: at least one fine wire anode; a cathode comprising a plurality of co-extensively juxtaposed, spacedapart, wafer-like apertured plates in stacked array with their apertures and their outer peripheries in alignment and with said anode extending centrally within an aligned row of apertures in said array, said cathode having a relatively large area of electrode surface in cooperative spaced relationship with the very small area of the anode, whereby upon establishment of any potential difference between the cathode and anode a great preponderance of the electric field provided thereby is concentrated in a region near to the anode and the field gradients near to said surface of the cathode are relatively small even for large applied potentials and large total gradients; an auxiliary electrode comprising a tubular reflector surrounding the array of cathode plates adjacent to and in cooperative spaced relationship with the cathode; said auxiliary electrode being positioned with its length-dimension extending substantially parallel to the anode; and conductive means for applying between said cathode and said auxiliary electrode a potential of much smaller magnitude than suitable cathode-to-anode potentials for Geiger-Mueller tubes for providing therewith greatly increased field gradients near to said cathode surface.

2. A detector as in claim 1 in which each of said plates has a plurality of apertures and each aperture of each plate is in alignment with a corresponding aperture of each of the other plates whereby there are as many aligned rows of apertures in the entire array as there are apertures in each plate, said anode extends centrally within one of the aligned rows of apertures; and said detector further comprises one additional fine wire anode for each additional aligned row of apertures, each of the additional anodes extending centrally within a respective one of the additional rows of apertures.

3. A detector as in claim 2 in which said detector further comprises as additional auxiliary electrodes an additional foraminous tubular element for each addi tional aligned row of apertures, each of the additional tubular elements being positioned with respect to a respective additional anode and a respective additional row of apertures like the first-mentioned tubular elern w h espa o Said firshmemipn d ode and said first-mentioned row of apertures, v v

4. .A. radiation detector as in claim 1 which further bompris la' e n a i r lfi rb e a ps a u ul second auxiliary electrode comprises a tubular foraminous member.

6. A radiation detector as in claim 4 wherein said seeond auxiliary electrode comprises a plurality of rings individually surrounding and spaced from the anode.

Rams s cm nin fil f hi 9mm STATES PATENTS r -a Ap a-2 1945 a Aug. 30, 1949 Hare a-z- Fe r u 7 QTHER REFERE CES 'Use of a Grid to Reduce Operating Voltage in G=M Counters, K011i et a1. Rev. of Sei, Inst. August 1940, vol. 11 pg's. 267-269. V 

1. A RADIATION DETECTOR OF THE GEIGER-MUELLER TYPE COMPRISING: AT LEAST ONE FINE WIRE ANODE; A CATHODE COMPRISING A PLURALITY OF CO-EXTENSIVELY JUXTAPOSED, SPACEDAPART, WAFER-LIKE APERTURED PLATES IN STACKED ARRAY WITH THEIR APERTURES AND THEIR OUTER PERIPHERIES IN ALIGNMENT AND WITH SAID ANODE EXTENDING CENTRALLY WITHIN AN ALIGNED ROW OF APERTURES IN SAID ARRAY, SAID CATHODE HAVING A RELATIVELY LARGE AREA OF ELECTRODE SURFACE IN COOPERATIVE SPACED RELATIONSHIP WITH THE VERY SMALL AREA OF THE ANODE, WHEREBY UPON ESTABLISHMENT OF ANY POTENTIAL DIFFERENCE BETWEEN THE CATHODE AND ANODE A GREAT PREPONDERANCE OF THE ELECTRIC FIELD PROVIDED THEREBY IS CONCENTRATED IN A REGION NEAR TO THE ANODE AND THE FIELD GRADIENTS NEAR TO SAID SURFACE OF THE CATHODE ARE RELATIVELY SMALL EVEN FOR LARGE SUPPLIED POTENTIALS AND LARGE TOTAL GRADIENT; AN AUXILIARY ELECTRODE COMPRISING A TUBULAR REFLECTOR SURROUNDING THE ARRAY OF CATHODE PLATES ADJACENT TO AND IN COOPERATIVE SPACED RELATIONSHIP WITH THE CATHODE; SAID AUXILIARY ELECTRODE BEING POSITIONED WITH ITS LENGTH-DIMENSION EXTENDING SUBSTANTIALLY PARALLEL TO THE ANODE; AND CONDUCTIVE MEANS FOR APPLYING BETWEEN SAID CATHODE AND SAID AUXILIARY ELECTRODE A POTENTIAL OF MUCH SMALLER MAGNITUDE THAN SUITABLE CATHODE-TO-ANODE POTENTIALS FOR GEIGER-MUELLER TUBES FOR PROVIDING THEREWITH GREATLY INCREASED FIELD GRADIENTS NEAR TO SAID CATHODE SURFACE. 